Well, you could simply let it rest on a 1" dowel. But that might interfere with the ability of the windmill's spinning, depending on how deep the sails' shaft goes into the housing. If letting the entire thing sit on a 1-inch dowel does interfere with the operation of the sails, then you could put 3 - 4 stainless steel screws into the dowel about 3 inches from it's end, they will give the base of the windmill something to rest against. BUT, that means a good wind could cause the entire thing to spin.

Ultimately, what I would personally do is drill a 1-inch hole at the end of a 4x4, squirt a bit of wood glue into the hole and push a 1-inch dowel into that hole. Cut the dowel so that about 2 - 3 inches is left above the 4x4 and then apply some wood glue before sliding the windmill onto the dowel.

Thank you for ideas, Leekle! I was considering the first suggestion but never thought of the whole thing spinning with the wind. Good point! The second option sounds like it will work the best. The only problem is I was hoping to bring the windmill inside for winter months, but if I glue it to the dowel, it will be a permanent structure. Can you think of any way to make it semi-permanent? Like to drill another hole in the bottom of 4x4 and let it sit on some kind of steady rode (PVC pipe or similar) installed deep in the ground? Then I could take the mill and 4x4 inside. Would it spin on the rode? I can probably prop 4x4 with something heavy like rocks. Thanks!

To keep it portable so you can take it inside, you could use some hook and loop tape stapled to the top of the 4x4 and the bottom of the windmill. It won't be super sturdy, but between the dowel and the tape, it should be able to stand up to fairly stiff winds. I would probably grab it and stash it if any severe thunderstorms come rolling in.

Thank you, Leekle! The first impulse was to put the windmill in place protected from the wind, but then it kind of defies its purpose - to spin in the wind.
I might go with your suggestion to glue to the dowel on 4x4 and then I can use a wrap for the winter, like for a fountain. Thanks again!

Another thought I just had... you can drill a smaller 1/4" hole in the end of the 4x4 which would be offset from the center 1" dowel and push a 1/4" dowel into it. You could then drill another 1/4" hole in the base of the windmill at the same offset. This will create a 'key' for the windmill to sit on. You could even drill 4 - 1/4" holes in the base of the windmill, one for each cardinal direction (north, south, east, west). This would let you turn the windmill to another direction if you so chose to, but it would prevent the wind from turning it.

Great idea! Simple and effective. Thank you so much, Leekle! Windmill will be steady and portable at the same time. Problem solved. I really appreciate your help and a brilliant advice.